In the manufacture of paper an extremely dilute mixture of fibers, water, and some additives is poured on an endless screen that is of such fine mesh size that the liquid phase can be sucked and pressed through it, leaving a uniform layer of fibers on the screen. When the layer of fibers and the screen pass between high-pressure dewatering rollers the screen is subjected to enormous (=/&gt;100 bar) pressures transversely and it is also subjected to fairly considerable longitudinal tensions since it must be held taut and free of vibration.
Such a screen is made of a strip of material whose ends are joined. Thus in use they are endless, while at the same time they can be threaded through the machines when they need to be replaced. This joint must provide the solid connection needed to resist longitudinal tension, must permit the same runoff or runthrough of liquid as the rest of the screen, must present a surface like that of the rest of the screen, and must be relatively easy to open and close. In addition the mass of the screen should be about the same at the joint as anywhere else on the screen so that if heated dewatering and drying rollers are used the joint will not form a cold spot.
In one such system, one end of each warp filament is formed into a loop lying in a longitudinal plane perpendicular to the plane of the screen at the joint and these loops are interleaved. A joint rod extending parallel to the weft filaments engages through the interleaved loops. The opposite end of each warp filament terminates short of the respective looped end at the joint in longitudinal line with this looped end. Such an joint is fairly weak with regard to longitudinal forces since only half of the ends of the warp filaments are actually engaged at the joint. Thus at the joint the longitudinal resistance to breaking will be half that of the rest of the screen.
In the system of French Pat. No. 2,195,303 one end of each warp filament is formed with a long loop and the opposite end with a short loop. The ends are aligned with the long loop of each warp filament lying transversely against and longitudinally overlapping the short loop of the same warp wire, and two longitudinally spaced joint rods are fitted to the assembly, with the one rod going through the loops of half of the warp filaments and the other rod going through the loops of the other warp wires. Such an arrangement can withstand substantial longitudinal forces, but constitutes a dense spot that water cannot pass transversely through or longitudinally along as well as through and along the rest of the screen. Furthermore the joint region also constitutes a hard spot in the screen that can cause the dewatering press to mark the paper pressed against it.
In U.S. Pat. No. 4,095,622 the screen has warp filaments that are generally continuous over the belt. Long loops and short loops are formed here by the joining of two adjacent warp ends so that these loops all inherently lie in a plane mainly parallel to that of the screen. They can be interleaved and connected by two joint rods, an operation that requires meticulous twisting of the screen and working of the rods through the loops. Since each rod only traverses two-thirds of the loops, the joint is fairly weak and the crimp where they cross over these rods can damage the warp filaments.